416 CHAPTER 9. SOLITON SYSTEMS
9.2.4 Soliton Transmitters
Soliton communication systems require an optical source capable of producing chirp-
free picosecond pulses at a high repetition rate with a shape as close to the “sech”
shape as possible. The source should operate in the wavelength region near 1.55μm,
where fiber losses are minimum and where erbium-doped fiber amplifiers (EDFAs) can
be used for compensating them. Semiconductor lasers, commonly used for nonsoliton
lightwave systems, remain the lasers of choice even for soliton systems.
Early experiments on soliton transmission used the technique of gain switching for
generating optical pulses of 20–30 ps duration by biasing the laser below threshold and
pumping it high above threshold periodically [38]–[40]. The repetition rate was de-
termined by the frequency of current modulation. A problem with the gain-switching
technique is that each pulse becomes chirped because of the refractive-index changes
governed by the linewidth enhancement factor (see Section 3.5.3). However, the pulse
can be made nearly chirp-free by passing it through an optical fiber with normal GVD
(β 2 >0) such that it is compressed. The compression mechanism can be understood
from the analysis of Section 2.4.2 by noting that gain switching produces pulses with a
frequency chirp such that the chirp parameterCis negative. In a 1989 implementation
of this technique [39], 14-ps optical pulses were obtained at a 3-GHz repetition rate by
passing the gain-switched pulse through a 3.7-km-long fiber withβ 2 =23 ps^2 /km near
1.55μm. An EDFA amplified each pulse to the power level required for launching
fundamental solitons. In another experiment, gain-switched pulses were simultane-
ously amplified and compressed inside an EDFA after first passing them through a nar-
rowband optical filter [40]. It was possible to generate 17-ps-wide, nearly chirp-free,
optical pulses at repetition rates in the range 6–24 GHz.
Mode-locked semiconductor lasers are also suitable for soliton communications
and are often preferred because the pulse train emitted from such lasers is nearly chirp-
free. The technique of active mode locking is generally used by modulating the laser
current at a frequency equal to the frequency difference between the two neighboring
longitudinal modes. However, most semiconductor lasers use a relatively short cavity
length (< 0 .5 mm typically), resulting in a modulation frequency of more than 50 GHz.
An external-cavity configuration is often used to increase the cavity length and reduce
the modulation frequency. In a practical approach, a chirped fiber grating is spliced
to the pigtail attached to the optical transmitter to form the external cavity. Figure
9.8 shows the design of such a source of short optical pulses. The use of a chirped
fiber grating provides wavelength stability to within 0.1 nm. The grating also offers
a self-tuning mechanism that allows mode locking of the laser over a wide range of
modulation frequencies [41]. A thermoelectric heater can be used to tune the operat-
ing wavelength over a range of 6–8 nm by changing the Bragg wavelength associated
with the grating. Such a source produces soliton-like pulses of widths 12–18 ps at a
repetition rate as large as 40 GHz and can be used at a bit rate of 40 Gb/s [42].
The main drawback of external-cavity semiconductor lasers stems from their hy-
brid nature. A monolithic source of picosecond pulses is preferred in practice. Several
approaches have been used to produce such a source. Monolithic semiconductor lasers
with a cavity length of about 4 mm can be actively mode-locked to produce a 10-GHz
pulse train. Passive mode locking of a monolithic distributed Bragg reflector (DBR)